1. Field of the Invention
The present invention relates to an image forming apparatus such as a color copying machine, a color laser printer, or the like.
2. Related Background Art
An example of a conventional tandem type image forming apparatus is shown in FIG. 16.
The image forming apparatus 401 includes a transferring belt 405 being a transferring material bearing body for bearing and conveying a material to which an image is transferred (hereinafter referred to as a transferring material P), and process cartridges (hereinafter simply referred to as xe2x80x9ccartridgesxe2x80x9d) for yellow Y, magenta M, cyan C and black Bk 414, 415, 416 and 417 are arranged in tandem along the transferring material bearing surface of the transferring belt 405. Above the cartridges 414 to 417, optical units 418, 419, 420 and 421 are disposed correspondingly to each of the cartridges 414 to 417. Moreover, transferring rollers 410, 411, 412 and 413 are disposed in a state of being correspondingly opposed to photosensitive drums 406, 407, 408 and 409 being image bearing bodies of the respective cartridges 414 to 417 with the transferring belt 405 put between them.
In the aforesaid structure, toner images of yellow, magenta, cyan and black obtained by being processed by well known electrophotographic processes are transferred one over another on the transferring material P fed on the transferring belt 405 by a pickup roller 403 and feeding/conveying rollers 429 from a sheet cassette 402, and the toner images are fixed by a fixing unit 422. And then, the transferring material P is delivered to the outside of the image forming apparatus 401 through a delivery sensor 424 and a paper path 423.
Moreover, when a toner image is formed also on the backside of the transferring material P, the transferring material P is again conveyed to the transferring belt 405 through another paper path 425 after exiting the fixing unit 422, and then the image is formed also on the backside after undergoing similar processes.
Incidentally, the transferring belt 405 is driven to rotate by a transferring belt driving roller 404.
Moreover, the optical units 418 to 421 of each color expose to scan the surface of each of the photosensitive drums 406 to 409 with laser beams L1, L2, L3 and L4 for forming an latent image, respectively, and a series of these image-forming operations are controlled to scan the surfaces of the photosensitive drums 406 to 409 in order that images are transferred from a predetermined position on each conveyed transferring material P by being synchronized with one another.
Furthermore, the image forming apparatus 401 includes a feeding motor for driving the feeding/conveying rollers 429, a transferring belt driving motor for driving the transferring belt driving roller 404, a drum driving motor for driving each color photosensitive drum 406 to 409, and a fixing roller driving motor for driving fixing rollers 422a in the fixing unit 422. For the acquisition of a good quality image, these motors are controlled to rotate at a fixed number of revolutions.
However, the conventional image forming apparatus 401 has a problem of the remarkable deterioration of the image quality of a formed image that is caused by the following reason. That is, the controlled temperature of a heater built in the fixing unit 422 and the heat evolution by each driving motor make the temperature in the image forming apparatus 401 rise. The rise of the temperature causes the thermal expansion of the transferring belt driving roller 404, which makes the rate of the transferring belt 405 faster. Then, the so-called color aberration is produced when each color toner image is transferred from a specific position on a transferring material P one over another. Namely, the color aberration brings about the remarkable deterioration of image quality. That is, because the photosensitive drums 406 to 409 and the transferring belt driving roller 404 are controlled to rotate at a fixed rate, the peripheral velocity of the transferring belt 405 becomes faster when the diameter of the transferring belt driving roller 404 becomes large. That causes the color aberration.
As a measure for resolving such a problem, there is a method of performing registration correction. The method is performed as follows. That is, a pattern for color aberration detection is formed on the transferring belt 405. The pattern is read in with a sensor to detect the amount of the relative color aberration of each color. On the detection results, the position at which the writing of each color image with the laser beam of each color L1 to L4 starts is corrected. However, the method has the following problems.
(1) Although the image writing starting positions immediately after the registration correction can be accorded to one another, in the case where temperature rise in the image forming apparatus 401 becomes large, for example, in case of continuous printing, the peripheral velocity of the transferring belt 405 gradually becomes faster, and the amount of the color aberration becomes large after the processing for a predetermined number of sheets.
(2) For resolving the problem, for example, an idea to perform the registration correction each performance of the printing of a certain fixed number of sheets has been proposed. However, the more frequently the registration correction is performed, the less the throughput of the image forming apparatus 401 becomes.
The resist correction also has the following problem. That is, because a pattern for registration correction is formed on the transferring belt 405 at a time of the registration correction, the consumption amount of toner becomes large, which makes the economical efficiency of the image forming apparatus 401 deteriorate for a user.
Moreover, as another measure of the correction of the color aberration, a method is disclosed in Japanese Patent Application Laid-Open No. 2000-071522. In the method, registration reference marks are formed on the transferring belt 405 in advance, and the reference marks are detected with a charge coupled device (CCD) sensor. The method corrects the image writing starting positions on the results.
However, this method requires to form the reference marks on the transferring belt 405 in advance, which brings about the problems such that the manufacturing costs of the transferring belt 405 rise and the width of the image forming apparatus 401 becomes large for securing the spaces for forming the reference marks.
Such problems are produced in an image forming apparatus equipped with an intermediate transferring body.
Furthermore, the conventional image forming apparatus includes the feeding/conveying rollers 429 for feeding and conveying the transfer material P. In this case, when the peripheral velocity of the transferring belt 405 becomes faster as the temperature rise in the image forming apparatus 401, the difference between the transferring material conveyance force of the feeding/conveying rollers 429 and the transferring material conveyance force of the transferring belt 405 becomes large to produce color aberration and image blurring. That is, when the transferring material conveyance force of the feeding/conveying rollers 429 becomes larger than the transferring material conveyance force of the transferring belt 405, the inclination of pushing the transferring material P into the conveyance direction becomes large. In such a case, when the transferring material P is a stiff transferring material such as a thick paper or the like, image blurring is produced in the trailing edge part of the transferring material P.
On the other hand, when the transferring material conveyance force of the transferring belt 405 is larger than the transferring material conveyance force of the feeding/conveying rollers 429, there is a problem such that image blurring or color aberration is produced at the leading edge part of the transferring material P.
Accordingly, one object of the present invention is to provide an image forming apparatus that can decrease color aberration and image blurring accompanying the temperature rise in the image forming apparatus to obtain high quality images while escaping the rise of costs and the increase of the size thereof. Still another object of the present invention is to provide an image forming apparatus comprising, conveying means for conveying a sheet, image forming means for forming an image on the sheet conveyed by the conveying means, reading means for reading the image on a surface of the sheet on a conveying path of the conveying means, detecting means for detecting a rate of the sheet on a basis of the image read by the reading means, and controlling means for controlling a conveying rate of the conveying means on a basis of the rate detected by the detecting means.
Other objects, configurations and advantages of the present invention will be apparent by the following detailed description and the attached drawings.